Sheeting and chunking have been a problem in commercial, gas phase polyolefin production reactors for many years. The problem is characterized by the formation of solid masses of polymer on the walls of the reactor. These solid masses of polymer (the sheets) eventually become dislodged from the walls and fall into the reaction section, where they interfere with fluidization, block the product discharge port, and usually force a reactor shut-down for cleaning, any one of which can be termed a “discontinuity event,” which in general is a disruption in the continuous operation of a polymerization reactor. The terms “sheeting,” “chunking,” and/or “fouling” while used synonymously herein, may describe different manifestations of similar problems, in each case they can lead to a reactor discontinuity event.
There are at least two distinct forms of sheeting that occur in gas phase reactors. The two forms (or types) of sheeting may be described as wall sheets or dome sheets, depending on where they are formed in the reactor. Wall sheets are formed on the walls (generally vertical portions) of the reaction section, such as walls 110 in FIG. 1. Dome sheets are formed much higher in the reactor, on the conical section of the dome, or on the hemi-spherical head on the top of the reactor, such as near the “dome” section 114 of the reactor shown in FIG. 1.
When sheeting occurs with metallocene catalysts, it generally occurs in either the lower section of the reactor and is referred to as wall sheeting, and/or it occurs near the dome of the reactor and is referred to as dome sheeting, e.g., both wall sheeting and dome sheeting can occur when using a metallocene catalyst.
Dome sheeting has been particularly troublesome with metallocene catalyst systems. Typical metallocene compounds are generally described as containing one or more ligands capable of bonding to a transition metal atom, usually, cyclopentadienyl derived ligands or moieties, in combination with a transition metal selected from Group 4, 5, or 6 or from the lanthanide and actinide series of the Periodic Table of Elements.
One characteristic that makes it difficult to control sheeting with metallocene catalysts is their unpredictable static tendencies. For instance, European Pat. No. EP0811638A2 describes metallocene catalysts as exhibiting sudden erratic static charge behavior that can appear after long periods of stable behavior.
As a result of the reactor discontinuity problems associated with using metallocene catalysts, various techniques have been developed that are said to result in improved operability. For example, various supporting procedures or methods for producing a metallocene catalyst system with reduced tendencies for fouling and better operability have been discussed in U.S. Pat. No. 5,283,218, which discloses the prepolymerization of a metallocene catalyst. U.S. Pat. Nos. 5,332,706 and 5,473,028 disclose a particular technique for forming a catalyst by “incipient impregnation.” U.S. Pat. Nos. 5,427,991 and 5,643,847 disclose the chemical bonding of non-coordinating anionic activators to supports. U.S. Pat. No. 5,492,975 discloses polymer bound metallocene catalyst systems. U.S. Pat. No. 5,661,095 discloses supporting a metallocene catalyst on a copolymer of an olefin and an unsaturated silane. U.S. Pat. No. 5,648,308 discloses removing inorganic and organic impurities after formation of the metallocene catalyst itself. PCT Pub. No. WO97/15602 discloses readily supportable metal complexes. U.S. Pat. No. 6,225,423 discloses forming a supported transition metal compound in the presence of an unsaturated organic compound having at least one terminal double bond.
Various methods for controlling sheeting have been developed. These methods often involve monitoring the static charges near the reactor wall in regions where sheeting is known to develop and introducing a static control agent into the reactor when the static levels fall outside a predetermined range. For example, U.S. Pat. Nos. 4,803,251 and 5,391,657 disclose the use of various chemical additives in a fluidized bed reactor to control static charges in the reactor. A positive charge generating additive is used if the static charge is negative, and a negative charge generating additive is used if the static charge is positive. The static charge in the reactor is typically measured at or near the reactor wall at or below the site where sheet formation usually occurs, using static voltage indicators such as voltage or current probes or electrodes.
U.S. Pat. No. 6,548,610 describes a method of preventing dome sheeting (or “drooling”) by measuring the static charge with a Faraday drum and feeding static control agents to the reactor to maintain the measured charge within a predetermined range. U.S. Pat. No. 6,548,610 also discloses the use of conventional static probes, such as those described in U.S. Pat. Nos. 6,008,662, 5,648,581, and 4,532,311. Other background references include WO 99/61485, WO 2005/068507, EP 0811638A, EP 1106629A, and U.S. Pat. Appl. Pub. Nos. 2002/103072 and 2008/027185.
Others have discussed different process modifications for improving reactor continuity with metallocene catalysts. There are various other known methods for improving operability including coating the polymerization equipment, controlling the polymerization rate, particularly on start-up, and reconfiguring the reactor design and injecting various agents into the reactor.
Although various methods have been developed to manage sheeting problems with metallocene catalysts and use of continuity additives has been investigated, the problem persists. One reason the problem persists is that the use of continuity additives can be accompanied by decreased catalyst efficiencies and productivities. Decreased catalyst efficiencies and catalyst productivities may occur where additives injections are not matched precisely in regards to frequency and/or amount to arrest transient instances of reactor static, which can presage undesirable “reactor discontinuity events.”
As can be seen from all of these patents directed toward improving polymerization reaction operability in gas-phase reactor systems, there is a need for improving the polymerization reaction process in gas-phase reactors when using metallocene catalysts of all types. Thus, it would be advantageous to have a polymerization process utilizing metallocene catalysts, the process being capable of operating continuously with enhanced reactor operability (defined as the general absence of sheeting or chunks that might lead to reactor discontinuity events) while refraining from depressing catalyst productivities.